Need to know things like PH levels, if you fert with ever watering or not, ect. Take PH readings from the top of the soil, as well as the bottom. Also take PH of water before watering, then as watter comes out of the bottom of the plant, take that PH as well. Post this info and you will get more accurate response. I dont want to tell you exactly what it is with out all the info, could get it wrong and mess up your plants.

That being said this is just a broad guess.... Its does not look like N deficiency, but does resemble Magnesium deficiency. Also could be a iron deficiency, but like i said with out the extra info, its hard to tell. But the way the yellow is only on the older leaves, and moving from the out side inward, (the stem is still green) it looks like one of the problems I listed above. It also could be nute lock out, which would cause the deficiency in the first place. PH problems and salt build up can cause the plant to stop taking in certain nutrients, resulting in deficiencies. Your new growth looks fine, so thats a good sign that you can easily fix this problem. Hope this helps, try and get some of the PH readings to see whats going on in the soil. And remember, the top of the soil can be very different from the bottom, so take PH on both.